(1) Field of the Invention
The present invention relates to an apparatus, and method for so using, ultrasonic and RF signals to establish the position of a moveable platform. More specifically, the present invention relates to a method of situating transceiver and transponder modules so as to measure the position of an elevator car in operation.
(2) Description of Related Art
A Positioning Reference System (PRS) is a component of an elevator control system that provides fast and accurate position measurement of elevator car in a hoistway. Many existing PRSs are based on encoders that are attached to the elevator motor, governor, or independent sheaves. These PRSs suffer from differences between the encoder reading and the real position that is caused by slippage, rope stretch, mechanical wear in subsystems, and/or building sway. To minimize the difference, correction should be performed frequently based on some fixed and known referencing points showing the real position of landing floor and leveling-zone. A vane system, consisting of vane reader and vanes, provides these referencing points and their detection means. Considering the simple functionality of the vane system, the vane system is quite cost-inefficient since a vane, which is installed at every floor by a mechanic in the hoistway, costs $10 for material, 0.5 hour for installation, and about 0.1 hour for adjustment. Overall, one of the most significant problems in the existing PRSs is the poor performance to cost ratio.
In response to the shortcomings of existing PRSs, there has been developed Passive Ultrasonic RF-ID Systems, in short, PURIS. However, PURIS systems pose additional challenges. For example, wireless power supply through ultrasound may not be sufficient to activate the transponders. In addition, aerodynamic interference may degrade the positioning performance significantly.
Each of these problems can be technically resolved by powering the transponder with RF as in RFID systems and using 4 transponders (PURIs) instead of 2 at every doorframe.
Although these two solutions are good enough, they may cost more than is necessary since the first solution needs a customized production of a solid-state RFID system and the second almost doubles the material cost of the resultant system. Wired power can easily solve the first problem. However, it still cannot solve the second problem in the PURIS framework.
What is therefore needed, is a high-accuracy positioning means with low cost for material, installation, and maintenance.